Large amounts of fossil fuels such as coal, oil, natural gas, and to some extent, biofuels, are used in power production, industrial processes, and transportation. Due to the increase in the use of these fuels, release of sulfur dioxides, nitrogen oxides, and carbon dioxide gases have also increased. In the atmosphere, these gases disperse widely and contribute to tropospheric photochemical smog, ozone formation, wet and dry acid disposition, stratospheric acid disposition, and the xe2x80x9cgreenhouse effectxe2x80x9d. Recently, great interest has been shown in oxides of nitrogen, that is, in nitric oxide (NO) and nitrogen dioxide NO2), which are jointly termed NOx, because they are pathogenic in humans, animals, and plants, and initiate noxious photochemical processes in the atmosphere.
Different measures are typically used in attempts to control or alleviate NOx, namely the so-called primary and secondary measures. In the primary measures, a number of combustion modifications may be made to reduce NOx emissions. There are many different possibilities of boiler modifications, such as low excess air, biased burner firing, over-fire air, flue gas recirculation, and the like, as well as any combination of these and other primary modifications.
When the limits of NOx emissions cannot be met with combustion control only, flue gas treatments systems, so-called secondary, or post-combustion, measures, may have to be implemented. Among these secondary measures, the dominant method in use for significant NOx reduction requirements is selective catalytic reduction (SCR). In many instances, an SCR installation may provide NOx reduction as high as 80-90%, or more.
In the SCR method the NOx concentration in the flue gas is reduced through injection of ammonia and passing the flue gas through a catalyst. The role of the catalyst and the reaction mechanism with the added ammonia results in the production of the products which are harmless to the environment. The SCR reaction is selective, which means the oxidation of ammonia and sulfur dioxide should not occur.
The efficiency of SCR system is dependent upon several factors, such as NOx, concentration at the inlet of the catalyst, the flue gas temperature and composition, the ratio of injected ammonia injection to NOx concentration, and catalyst relative size and properties such as space velocity, catalyst activity, pitch (in the case of honeycomb catalyst) and active catalytic area. Furthermore, a number of other factors, such as the chemical formulation of the catalyst, the type and chemical composition of the fuel being burned, add-mixtures being used for boiler conditioning or deslagging requirements, ammonia dispensing techniques, and the like, also have very significant effects on the efficiency and life of the catalyst being used in any SCR process. In other words, the nature of the SCR system, coupled with the flyash and other physical contaminants in the flue gas stream, can cause the grid or open area for flue gas flow through the catalyst to be reduced or completely closed. This reduction in flue gas flow through the catalyst modules in turn will result in inefficiencies of NOx removal, increased ammonia slip, additional system back pressure and, often, and premature failure of the SCR system.
Historically, cleaning of the SCR system was accomplished using known types of steam sootblowers. While steam sootblowers, which can be viewed as mechanical in nature, can indeed remove debris from the exposed walls of the catalyst substrate, they suffer from high capital costs, coupled with high operating and maintenance costs. Furthermore, because the steam or air and water is applied under pressure, the catalyst coating may experience significant premature wear, thus decreasing the useful life of the catalyst
In an effort to overcome the known deficiencies of sootblowers in the cleaning of SCR""s, an alternative cleaning arrangement has been gaining some degree of success. This emerging technology is the use of sound emitters positioned in a variety of locations. While sonic cleaning has met with limited success; however, the selection of emitter frequency, as well as the location, positioning, phasing and aiming of the emitters has been a hit or miss affair, and applied in a brute manner, rather than with the sophistication of a thought out scientific approach. Furthermore, prior applications of sonic cleaning in SCR applications used primarily high frequency emitters for cleaning because of the favored velocity effects; however, inasmuch as the high frequency waves have a relatively short wave length, and a higher rate of dissipation in the SCR substrate, the cleaning effect throughout the length of the catalyst layer has often been less than optimal.
Still further problems in the application of prior art sonic cleaning devices concerned the positioning of such emitters in a transverse direction to the flue gas flow. In such instances, there was no recognition in the prior art of the loss of cleaning effect because of the identical or near identical frequencies and phases of emitters in adjacent layers (i.e. the pulsating velocities of spaced layers of emitters would have a tendency to cancel out the cleaning effect in the catalyst layer between them).
The present invention relates to aiding in the rectifying or, in the least, alleviating the accumulation of flyash on the exposed surfaces of the catalyst.
The present invention provides a method and apparatus for cleaning, in a new and novel manner, catalyzed surfaces of a plurality of catalyst elements placed within SCR reactors.
More specifically, in one aspect of this invention, a plurality of sonic emitters are used, some of which emit high frequency and others of which emit low frequency sound waves. The improvement provided by this aspect of the present invention is that the low frequency sound waves have longer wave lengths which more or less lightly penetrate and sweep the entire depth of the SCR modules, and the more powerful, but shorter wave length of the high frequency emitters, can then be more effective for a more vigorous sweeping of the module surfaces.
The invention herein additionally includes the concept of optimizing cleaning by changing the phases of symmetrically located emitters in adjacent layers so that they are in counter phase with respect to each other. Such an arrangement will prevent the effect seen in the prior art of cancellation of the cleaning effect on the catalyst layers between spaced sets of emitters oriented transversely to the flue gas flow.
The invention herein also includes the creation of system design and positioning concepts which will permit the modeling and selection of optimal frequency, phase, and wave selection criteria.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment which, taken in conjunction with the accompanying drawings, illustrate, by way of example, the principles of the invention.